This application claims the priority of German Patent Application No. 197 35 318.5, filed Aug. 14, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and arrangement for checking the operability of a secondary air pump in an internal-combustion engine in a motor vehicle using an electronic control unit.
Such a process is known, for example, from German Patent Document DE 43 43 639 A1. The need for such a process is based on an arrangement which blows fresh air into the exhaust gas duct between the internal-combustion engine and the catalyst in a motor vehicle for bringing the catalyst to the operating temperature as quickly as possible. By means of the exothermal reaction between the exhaust gas and the blown-in fresh air in the exhaust gas duct, an accelerated heating of the catalyst occurs. In order to keep the pollutant emissions as low as possible, it is endeavored to heat the catalyst to its operating temperature as fast as possible. For this purpose, a properly operating secondary air pump is required. In the case of the process known from German Patent Document DE 43 43 639 A1, the operability of the secondary air system, including the secondary air pump, is judged by means of the air flow rate (secondary air flow) generated by the secondary air system. For this purpose, a quantity which characterizes the secondary air flow, among others, is determined from the air/fuel ratio of the mixture taken-in by the internal-combustion engine and from the air/fuel ratio existing downstream of the lead-in of the secondary air pipe in the exhaust gas duct. For determining the air/fuel ratios, a lambda probe is used which is already present in any event. The known process for checking the operability of the secondary air system can therefore not be started before the lambda probe is operable. However, normally the lambda probe only becomes operable once the blowing-in of fresh air by means of the secondary air pump for heating the catalyst is no longer required. As quickly as a few seconds after a cold start of the internal-combustion engine, as a result of the switched-on secondary air pump, the HC-fraction and the Co-fraction of the exhaust gases is reduced to such an extent that the secondary air pump could already be switched off. According to the known process, a longer running time of the secondary air pump is therefore required in order to be able to carry out only the secondary air pump diagnosis. However, a longer running time of the secondary air pump, on the one hand, causes an increased NOx-fraction in the exhaust gas and, on the other hand, a considerably reduced service life of the secondary air pump.
It is an object of the invention to provide a process for checking the operability of a secondary air pump in the case of an internal-combustion engine, which process is independent of the operational readiness of the lambda probe.
This object is achieved by a process for checking the operability of a secondary air pump in the case of an internal-combustion engine in a motor vehicle via an electronic control unit. The control unit detects the rotational speed of the internal-combustion engine and the throttle valve angle in the intake system of the internal-combustion engine. Using a characteristic diagram stored in the control unit, as a function of the detected rotational speed and of the detected throttle valve angle, the rate of air flow into the cylinders of the internal-combustion engine is estimated. The overall rate of air flow is sensed by an air flow rate sensor. The overall rate is composed of the air flow into the cylinders of the internal-combustion engine and of the secondary air flow delivered by way of the secondary control pipe into the exhaust gas duct. The difference between the overall rate of air flow and the rate of air flow into the cylinders of the internal-combustion engine is then analyzed to check the operability of the secondary air pump.
An advantageous arrangement for implementing the process is characterized in that the inlet of the secondary air pipe in the form of a bypass in the intake system branches off behind the air flow rate sensor and in front of the throttle valve.
According to the invention, an electronic control unit, preferably the internal-combustion engine control unit, detects the rotational speed of the internal-combustion engine and the throttle valve angle in the intake system of the internal-combustion engine. Then, by means of a characteristic diagram stored in the control unit, as a function of the detected rotational speed and the detected throttle valve angle, the rate of air flow into the cylinders of the internal combustion engine is estimated. Furthermore, by means of a rate of air flow sensor present anyhow, the overall rate of the air flow is measured which is composed of the rate of air flow into the cylinders of the internal-combustion engine and of the rate of secondary air flow delivered by way of the secondary air pipe into the exhaust gas duct. Then, the difference between the overall rate of air flow and the rate of air flow into the cylinders of the internal-combustion engine is analyzed. Preferably, it is examined in this case whether the overall rate of air flow is larger than the rate of air flow into the cylinders of the internal-combustion engine.
An advantageous arrangement for implementing the process has a bypass to the intake system as the secondary air pipe, through which the secondary air flow is delivered by means of the secondary air pump from the intake system into the exhaust gas duct. The inlet of the bypass branches off behind the rate of air flow sensor and in front of the throttle valve in the intake system.
With the process according to the invention and with the arrangement according to the invention, the operability of the secondary air pump or of the secondary air system can be checked independently of the lambda probe so that it is not necessary to wait for the operational readiness of the lambda probe. The checking of the operability of the secondary air pump can therefore be carried out immediately after a cold start of the engine. As a result, the time period during which the secondary air pump is on is also only a function of the operating temperature of the catalyst. This results in a reduced operating period which also increases the service life of the secondary air pump.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.